RFR: 8370520: GenShen: Track and report on promotion failures [v2]

[William Kemper]

On Fri, 24 Oct 2025 20:33:20 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Should probably be ok to read these variables without lock.  We're only logging these results when evacuation is no longer happening.  Right?
>
> Now that we have this new log message, can we get rid of the "promotion failure messages" for individual objects?

We only take the heap lock there conditionally when we haven't yet "squelched" the log message.  We could change this to a `log_debug(gc, plab)` level message. The message is still useful when trying to understand the history and context for a how a thread became unable to promote. I'm not completely convinced there aren't still cases where a thread _should_ be able to promote but can't for some (unknown) reason.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27962#discussion_r2462150204